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FATTY SUBSTANCES  (GENERAL METHODS)

373

5. Saponification

The object of this operation is the scission of fats into their components,
i.e., into acids and glycerine (or, with waxes, higher alcohols). It is effected
as follows : In a conical flask or a porcelain dish, 20 grams of the fat are
heated on the water-bath, with frequent stirring, with 15 c.c. of 50% aqueous
caustic potash solution and 30-40 c.c. of 95% alcohol until the liquid becomes
homogeneous and clear, this usually occurring after about half an hour.

With substances either containing higher alcohols (wool fat, waxes) or
mixed with unsaponifiable substances (mineral oils, various extraneous
matters), a clear liquid is not, however, obtained, since the action of the
potash causes the separation of the higher alcohols, hydrocarbons and other
unsaponifiable substances, w7hich are usually insoluble under the conditions
employed. In such a case it is well in order to ensure complete saponifica-
tion to prolong the heating for an hour or more, with frequent shaking.
In some instances, for example, with wool fat or waxes, it is necessary to
carry out the reaction under a certain pressure. For this purpose, use is
made of a round-bottomed flask, closed with a stopper carrying either a
two-bulbed safety funnel charged with mercury or a right-angled tube
dipping 5-6 cm. below the surface of mercury in a beaker. In other cases,
for special investigations on non-saponifiable substances, saponification in
the cold is employed ; this is effected by dissolving the substance in ether
or petroleum ether, adding a considerable excess of alcoholic caustic potash
solution and shaking for a long time.

When the saponification is finished, the product (soap) may be used
for various purposes, such as the examination of the unsaponifiable sub-
stances or higher alcohols (see 19 : Unsaponifiable Substances), the deter-
mination of the glycerine (see Glycerine), or the separation of the fatty acids.

For the last purpose, the product of the saponification is first freed from
alcohol by prolonged heating on a water-bath, the residue being dissolved
in hot water, the hot aqueous solution shaken well with excess of dilute
sulphuric acid and left at rest on the water-bath until the fatty acids are
collected at the surface of the aqueous liquid in a clear layer. The water
above the fatty acids is then siphoned off and replaced by fresh hot water,
with which the acids are stirred, this washing being repeated three or four
times. Instead of siphoning off the water, the latter and the fatty acids
may be decanted on to a moist filter and the acids washed with hot water
on the filter itself until the filtrate ceases to give the reaction for sulphuric
acid. The washed fatty acids, in a flat dish, are kept for some time (about
an hour) in a steam-oven and then filtered through a dry filter. They are
then ready for various determinations, such as the melting and solidifying
points, acid number, iodine number, solid and liquid acids.

6. Behaviour towards Solvents

The ordinary solvents for fatty matters are ether, benzene, carbon
disulphide and tetrachloride, chloroform and petroleum ether. All fats
dissolve in these liquids (castor oil, however, is almost insoluble in petroleum
ether). In alcohol fatty substances are more or less soluble according toand examining the distillate.   The amount of volatile solvents and water, while the liquid remains clear.r.